1. Field of the Invention
This invention relates generally to anion exchange resins. More particularly, it relates to a class of anion exchange resins containing two different exchange sites with improved selectivity, sorptive capacity, and exchange kinetics.
2. Background of the Art
Groundwater contamination can be the result of, for example, solutions contaminated with radioactive metal ions leaking out of lagoons and burial pits. At Paducah, Ky. a plume has apparently seeped through the clay of the vadose zone into the sandy aquifer below. Under the oxidizing conditions in near-surface groundwaters, the principal form of the element Tc is the TcO.sub.4.sup.- anion (Pourbaix, M., Atlas of Electrochemical Equilibria, p. 24, Pergamon Press, Oxford, 1966), and it is water soluble and quite mobile in underground aquifers. The mobility of this species when coupled with a long half-life of 213,000 years makes the presence of pertechnetate in groundwater an environmental concern. A related problem exists at other sites where materials used in the processing of uranium or plutonium have apparently leaked out of containers into the ground.
Commercially available anion exchange resins are capable of removing TcO.sub.4.sup.- ion in the presence of typical anions found in groundwater (see for example Del Cul, G. D., et al., Sep. Sci. and Tech. 28:551, 1993). Further examples of anion exchange resins are disclosed in U.S. Pat. Nos. 2,591,573; 2,632,001; 2,725,361; 2,801,224; and 5,141,965. However, improved selectivity is desirable and can result in substantial cost savings. The pertechnetate anion has a high affinity for strong base anion exchange resins such as those made from quaternary amines. Thus, a resin with both a high selectivity for technetium as pertechnetate anion (TcO.sub.4.sup.-) over other anions commonly encountered in groundwater aquifers (e.g., chloride, nitrate, and sulfate), and a high anion exchange capacity is desired for efficient removal of pertechnetate from contaminated groundwater aquifers.
Some of the physical properties of a resin that are known to enhance selectivity for less hydrated anions such as pertechnetate (or iodide, perchlorate, or perrhenate) over other more hydrated anions such as nitrate include increasing the hydrophobicity and steric bulk of the alkyl groups surrounding the anion exchange site. Such a trend has been noted in for example, a paper by R. E. Barron and J. S. Fritz (Journal of Chromatography 284:13-25, 1984). However, increasing the alkyl chain length (and steric congestion) surrounding the anion exchange site results in a concomitant decrease in the ease of ion transport within the resin. Thus, the rate of exchange decreases (i.e., it requires longer times for the resin to come to equilibrium with the solution it is exposed to). The use of bulky amines also results in a lower degree of functionalization, as the increased steric congestion prevents all of the available resin sites from reacting with the amine. In fact, the total anion exchange capacity decreases with the increasing size of the trialkyl amine. Thus, a need exists for an anion exchange resin with high selectivity, high exchange capacity, and enhanced exchange kinetics.
To balance and optimize the competing properties of large, bulky tertiary amines (an increased pertechnetate selectivity versus a decreased rate of exchange and a decreased total anion exchange capacity), we have invented a class of resins which contain a mixture of both small and large amines. These resins having anion exchange sites composed of two separate and differently-sized amines possess higher sorptive capacities in a given amount of time for pertechnetate over resins possessing exchange sites derived from only one tertiary amine. The new materials improve on the problem of high selectivity but poor exchange capacity and kinetics characteristic of resins containing only one type of exchange site.